In radio communication systems operating with digital modulation arts amplifiers are required that provide multiple gain steps and that keep the noise figures of the amplified output low. Such Low Noise Amplifiers (LNA) also require input and output return losses independent of the gain step and high linearity also for the low gain modes. Controlling the gain of the Low Noise Amplifier by using current steering techniques shows the disadvantage of higher noise figures occurring in the low gain and medium gain modes than in the high gain modes. Throwing away some of the signal current by dumping to power supply, as is done in some designs of Low Noise Amplifiers for the low and medium gain modes, decreases the signal power, and hence degrades the noise figure by reducing the signal to noise ratio at the output of the Low Noise Amplifier. Cascade Low Noise Amplifiers change the current for the common base transistor by shunting and as a result the output return loss and the linearity degrade.
An approach has to be found that allows independent selection of the gain and the linearity of each state without effecting significantly the other parameters.